Master Gardeners: Some insects can spread diseases among plants

AT THIS TIME of year when mosquitoes are rapidly breeding, we frequently hear about the Marin/Sonoma Mosquito and Vector Control District responsible for the prevention, elimination or control of mosquitoes and other arthropods known to be potential carriers of infectious disease in people.

Master Gardeners write repeatedly about the idea that insects can often be our garden allies, but they can also act as disease vectors among plants. There is a lot of technical science and terminology involved in identifying and solving the myriad problems of plant diseases transmitted by insects. Let's cut it down to the basics for our everyday use at home.

For our purposes, a vector is an insect capable of introducing a pathogen into a plant that can cause disease in the plant. A pathogen is a bacterium, virus, fungus or anything that can cause disease in the host plant. The host plant is the plant upon which the vector feeds. So it follows that our host plant is in danger of "catching" a disease caused by the pathogen spread by the vector.

Plant diseases interfere with one or more of the working functions of the plant such as absorbing water or nutrients. This interference reduces the plant's ability to grow and produce the flowers, fruits, nuts, etc. we grow it for. Some of these diseases are caused by environmental factors such as nutrient deficiencies, soil moisture or temperature extremes. Often bacterial diseases are transmitted by rain, wind, soil or seed. Many of the harmful vectors are transmitted by insects either accidentally as they move from plant to plant or by a specific vector that moves a specific pathogen from one plant to another of the same species.

Insects transmit the pathogens in three main ways.

First, many insects transmit bacteria and fungal spores by feeding in or walking on a part of an infected plant that has the bacteria or spores on its surface. The bacteria and spores are often sticky and cling to the visiting insect. As the insect travels from plant to plant, it deposits the offenders to a new plant.

Second, some insects can transmit certain fungi, bacteria or viruses by feeding on an infected plant and carrying the pathogen on their mouthparts as they visit and feed on other plants.

Third, the pathogen can be transmitted internally within the digestive system and salivary glands of the vector.

Depending upon the involved pathogen, the vector can transmit the infection immediately after feeding on an infected host, but some require a period of incubation and circulation within the vector before they can be transmitted. Aphids, for example, are known to transmit many plant viruses immediately after feeding on an infected host plant. They fly from plant to plant making probing attempts to feed before they settle in on one plant to continuously feed and reproduce. Each plant that they visited can be infected quickly. The glassy-winged sharp-shooter transmits Pierce's disease, a bacterial disease which threatened the wine industry, by leaving droplets of their waste as they feed on the wide variety plants they find enticing.

Insect control is always a complicated issue. Often you need to know the life cycle of the pathogen and the vector. Sometimes the solution (such as insecticides) becomes a part of the problem. The best ways to stop the spread of disease by vectors are to limit the ability of the vector to acquire the pathogen or to prevent the infected vectors from transmitting the pathogen.

For us at home, conscientious garden observation, cleanliness and sanitation are our best defenses. Make sure any plant material you bring into your garden is disease and insect free. Watch for any signs of disease in your garden. Remove any diseased plant debris quickly and dispose of it in your green can. Keep your tools and gloves clean, especially if you have been working in an infected area, to prevent you unwittingly being the transmitting transport assistant. You can carry a spray bottle of alcohol or cleaning wipes around with you. When you prune, clean your tools between plants, or even between branches, if you suspect a disease.

The University of California Marin Master Gardeners are sponsored by UC Cooperative Extension. For questions about gardening, plant pests or diseases, call 473-4204 from 9 a.m. to noon, and 1 to 4 p.m. weekdays, or bring in samples or pictures to 1682 Novato Blvd., Suite 150B, Novato.

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For more information on insects that transmit plant pathogens, go to http://bit.ly/1g4Bg4V